The woman was taken to hospital in a critical condition, but is now stable in intensive care. She has not been identified out of respect for her family's privacy, the zoo said.

The elephant involved was a two-and-half-year-old male calf called Pathi Harn, which means "miracle" in Thai. The calf was given the name after being born alive, even after experts pronounced him dead in the womb following a difficult eight-day labor.

The calf -- initially referred to as "Mr Shuffles" due to his odd gait -- was conceived by artificial insemination as part of the zoo's elephant breeding and conservation program.

At the time of his birth in March 2010, the young male weighed 116 kilograms and one year later tipped the scales at just over 500kgs.

The zoo said it doesn't know what provoked the calf. "It's unknown at this time why the young elephant challenged the keeper. The public were not at risk at any time and the elephants are now in their paddock at the zoo."

WorkCover, a New South Wales government agency "promoting productive, healthy and safe workplaces," is investigating the incident.